x86/bugs: Allow retbleed=stuff only on Intel

The retbleed=stuff mitigation is only applicable for Intel CPUs affected
by retbleed.  If this option is selected for another vendor, print a
warning and fall back to the AUTO option.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/20250418161721.1855190-10-david.kaplan@amd.com